Silicon carbide (SiC) is a reinforced material having a high tensile rate. Alumina (Al2O3) is representative of oxide ceramics, and SiC is representative of non-oxide ceramics.
SiC has a wide range of applications due to its excellent mechanical properties such as high wear resistance, excellent thermal properties such as excellent high-temperature strength and high creep resistance, and excellent chemical resistance properties such as high oxidation resistance and high corrosion resistance and has been commonly used in mechanical seals, bearings, a variety of nozzles, high-temperature cutting tools, fireproof panels, abrasives, reducing agents for steel, and lightning arrestors.
However, since the preparation of a SiC sintered body is not simple and easy, the manufacturing cost of a SiC sintered body is very high, and thus, there is a clear limit in making the utmost use of a SiC sintered body. Reducing the manufacturing cost of a SiC sintered body is one of the most important challenges faced to fabricate a SiC sintered body.
To prepare a SiC sintered body, a sintering aid is inevitable. A yttria-alumina-based material, a metal/iron/aluminum mixture, a beryllium compound, a boron compound, or the like may be used as the sintering aid.
However, even if the sintering aid is used, a SiC sintered body having a practical use, e.g., a SiC sintered body having excellent physical properties at a temperature of about 1600° C., can only be fabricated by a long fabrication process performed at a very high temperature of about 2000° C. or higher.
A method of fabricating a SiC sintered body having excellent high-temperature strength through liquid-phase sintering has been disclosed (Kim et al. Acta Mater., 2007). According to this method, a sintered body is fabricated by performing sintering at a temperature of 2000° C. for 6 hours and using Sc2O3—Ru2O3—AlN as a sintering aid, and tensile strengths of 644 MPa and 600 MPa are measured from the sintered body at room temperature and a temperature of 1600° C., respectively.
SiC ceramics prepared by liquid-phase sintering performed at low temperature generally undergoes a considerable decrease in strength at a temperature of 1500° C. or lower. For example, in the case of using a Al2O3—Y2O3-based sintering aid, sintering is possible at a temperature of 1950° C., but a decrease in bending strength and strong plastic deformation are both observed at a temperature of 1400° C. (A. L. Ortiz et al., J. Europ. Ceram. Soc., 24, 3245-3249 (2004)).
SiC obtained by solid-phase sintering using B4C and C as a sintering aid maintains excellent strength at a temperature of up to 1500° C., compared to its strength at room temperature, but requires a sintering temperature of as high as 2150° C. for densification (G. Magnani et al., J. Europ. Ceram. Soc., 21, 633-638 (2001)).
That is, a SiC sintered body showing having excellent high-temperature properties, such as a less decrease in strength, at a temperature of up to 1500° C. generally requires very high sintering temperature and long sintering holding time.
Particularly, high sintering temperature means a considerable amount of energy, which leads to an increase in the manufacturing cost of a SiC sintered body. Accordingly, the development of a sintering aid capable of lowering sintering temperature while maintaining the physical properties of SiC is needed, and thus, the development of a SiC sintered body that can be sintered at low temperature and can be highly densified is also needed.
In recent years, research has been vigorously conducted on ways to fabricate a SiC sintered body having high electrical conductivity.
SiC having high electrical conductivity is expected to be used in various fields such as heating elements for high temperature, high-energy elements, and the like.
For example, studies show that specific resistance can be reduced to 1.8×10−4Ω·cm by adding TiN having high electrical conductivity in a second phase, in which case, however, the problem of residual stress may arise due to a difference in a thermal expansion coefficient with the second phase and there also is a disadvantage in that sintering needs to be performed at a high temperature of 2000° C. and a high pressure of 40 MPa for as long as 3 hours.
Thus, it is necessary to develop a SiC powder and a SiC sintered body that not only can be sintered and densified at a relatively low temperature and a relatively low pressure within a short period of time, but also have high electrical conductivity.
In the meantime, SiC has excellent mechanical properties. However, SiC requires the use of a considerable amount of diamond-based abrasives because of its high hardness, and increases in the price of parts, caused by high processing costs, are one of the main factors that inhibit the wide use of a SiC-based material.
To address these problems, various near-net shaping processes such as slip casting, gel casting, and freeze casting that are to be performed after the preparation of a high-concentration SiC slurry have been developed.
In order to manufacture a molded article with high strength, high density, and high uniformity through slurry processing, it is necessary to prepare a slurry having a high concentration and a high viscosity.
It has been reported that a high-concentration slurry of 60 vol % or higher can be obtained from other ceramic powders than SiC.
Studies show that for example, in the case of using Al2O3, a slurry having a concentration of as high as 62 vol % can be fabricated and in the case of using SiO2, a slurry having a high concentration of as high as 68 vol % can be fabricated.
Studies also show that in the case of using a SiC powder having a relatively coarse grain size of 0.6 μm, a slurry having a concentration of up to 57 vol %, which, however, is relatively low compared to slurries obtained by other ceramic powders, can be prepared.
This is because SiC has the highest Hamaker constant among ceramic materials and is thus affected by the strongest Van der Walls force when dispersed in water.
After comparing the Hamaker constants of various ceramic powders in water, it is noted that the Hamaker constants of Al2O3, β-Si3N4, SiO2, TiO2, and Y2O3 are 4.72, 6.57, 0.71, 5.65, and 3.85, respectively, but β-SiC has a Hamaker constant of 11.9, which is much higher than the Hamaker constants of the other ceramic powders (Bergstrom, L., Hamaker constants of inorganic materials, Adv. Colloid Interface Sci., 70, 125-169 (1997)).
Therefore, various studies have been conducted on methods to form SiO2 through the oxidation of the surface of powder or to form a thin coating layer on the surface of powder with Al2O3 and thus to disperse Si3N4 or SiC, which is relatively difficult to disperse.
These methods, however, inevitably involve adding impurities to a raw-material powder to improve dispersibility.
Thus, it is necessary to develop a technique for preparing a high-concentration SiC slurry with improved dispersibility.